Field of the Invention
The present invention relates to a self light emitting display device in which for example organic EL (electroluminescent) display elements are employed as light emitting pixels, and particularly to a self light emitting display device which is constructed in such a way that display images can be visually recognized on both front and rear faces of a display device and to information equipment employing this self light emitting display device.
Description of the Related Art
A display employing a display panel which is constructed by arranging light emitting elements in a matrix pattern has been developed widely. As the light emitting element employed in such display panel, an organic EL element in which an organic material is employed in a light emitting layer has attracted attention, and a display of a self-emitting type in which the organic EL elements are arranged in a matrix pattern has been commercialized partly. This is because of a background that by employing, in a light emission functional layer constituting an EL element, an organic compound which enables an excellent light emission characteristic to be expected, progress in a high efficiency and a long life by which an EL element can be fit for practical use has been made.
As a display panel employing the above-mentioned organic EL elements, a passive matrix type display panel in which EL elements are simply arranged in a matrix pattern and an active matrix type display panel in which respective active elements for example constituted by TFTs (thin film transistors) are added to respective EL elements arranged in a matrix pattern have been proposed. However, in either type, in order to drive and control respective light emitting elements (pixels), the light emission functional layer is sandwiched between a first electrode and a second electrode.
One electrode is formed for example by an ITO (indium tin oxide) film that is a light transmitting electrically conductive material so that light from the light emission functional layer is drawn to the outside via the electrode made of ITO. That is, this conventional type of self emitting display is constructed in such a way that images are reproduced on one surface side of the display.
Meanwhile, a board utilizing the EL light emitting board has been proposed for a guide display board which is arranged in a public facility such as a railroad station, and a double-sided self emitting type display device which can display different guide information for respective front and rear faces of the guide display board is disclosed in Japanese Patent Application Laid-Open No. 2000-58260 (paragraph 0020, FIG. 1).
The double-sided self emitting type display device disclosed in the Japanese Patent Application Laid-Open No. 2000-58260 has a form in which one-sided display type light emitting display panels conventionally known are stacked back to back so to speak, and in this case, a rear surface electrode for example constituted by a metal is shared for common use to make the display device thinner though not quite satisfactorily.
In a guide display board or the like arranged in a public facility or the like as described above, it is considered that since displaying guide information on both faces thereof is the main purpose and since the extent of requirement of making the display thinner is not so high, the rear surface electrode is shared to make the display thinner in accordance with the requirement. However, in portable type information equipment whose demand has become greater remarkably these days, for example, in a foldable cellular telephone, a display portion constituted by a relatively large-sized LCD which is used in an opened state as is commonly known and a display portion constituted by a small-sized LCD which is used in a closed state are arranged in a back-to-back state.
In a display device displaying information on both front and rear faces used in such foldable cellular telephone, further making it thinner is desired, and it has been a significant technical problem to further make it lighter in addition to making it thinner.